Advancement in the manufacturing of microelectronic circuits have been significant over the past three decades. Currently, millions of transistors are routinely fabricated into integrated circuits, such as, for example, those typically used as microprocessors. Microelectronic manufacturing techniques typically employ photolithography and electron beam lithography to perform most of the manufacturing of today's integrated circuits.
However, there are drawbacks associated with standard microelectronic manufacturing techniques. For instance, these manufacturing techniques are generally not capable of creating complex three-dimensional structures. Additionally, standard microelectronic manufacturing techniques are typically not able to form curved and/or uneven shapes. Furthermore, these fabrication techniques are incompatible with many chemical and biological environments.
A distinct manufacturing technology that is gaining recent popularity is two-photon polymerization (TPP). TPP has several advantages over standard microelectronic fabrication techniques. For instance, TPP manufacturing techniques provide for the creation of complex three-dimensional microstructures. These techniques also allow the production of geometries with essentially no topological constraints and with a dimensional resolution smaller than 100 nanometers (nm). Such complex three-dimensional microstructures may be coated with electrical conductors and/or semiconductors for uses in microelectronic and photonic applications. Additionally, TPP may have applications in the field of biomedicine, including drug delivery and tissue engineering.
TPP essentially entails a nonlinear interaction of light with a photosensitive material, such as resin. In particular, near-infrared photons are used to induce two-photon absorption in molecules (e.g., photoinitiators) in acrylic-based resin, beginning a polymerization process. In such process, highly cross-linked polymers are formed. More specifically, while an acrylic-based resin specimen is subjected to a TPP process, carbon-carbon double bonds are homolytically cleaved by action of radicals, forming multiple new carbon-carbon single bonds. The TPP process entails the use of an ultra-short pulsed laser and strong focusing lens to subject the specimen with the corresponding radiation to achieve very accurate geometries. The non-polymerized material of the specimen may be removed using solutions to leave the freestanding structure. It shall be understood that TPP may be applied to other types of resins and other materials.
The characterization, monitoring, and optimization of TPP processes are issues that are typically given considerable attention. For instance, attention is often given to how to characterize and measure the mechanical properties of specimens undergoing TPP processes. Also, attention is often given to how to improve or optimize a TPP process on a particular specimen. Additionally, attention is often given to how the solvent used to remove the non-polymerized material affects the remaining structure. Conventional inspection methods, such as bright-field transmission light microscopy and scanning electron microscopy (SEM), may not be able to accurately address these issues. For instance, bright-field transmission light microscopy may not provide sufficient detail to enable a three-dimensional view of the specimen. SEM may not provide sufficient detail about the structural information of the specimen.